In general, it is recognized that there is a strong correlation between the state of charge (SOC)[%] of a lead storage battery that shows an index of charged electricity in a battery and an open circuit voltage (OCV) which is measured assuming that the battery terminals are disconnected from a circuit. Therefore, it is most convenient to check the OCV in order to take a measurement of SOC, as proposed in U.S. Pat. No. 6,531,874 (JP-A-2002-22268) and JP-A-2002-250757.
However, a lead storage battery mounted on a vehicle (automotive battery) is always connected to an electric device regardless of the vehicle's condition (either running or parking). It is therefore impractical to disconnect the terminals of the battery from the connecting circuit. That is, taking a measurement of OCV of an automotive battery for the purpose of SOC calculation is not realistic.
Also, as shown in JP 2910184, it is proposed that an initial residual capacity is derived from the voltage-current characteristics in terms of a large capacity discharge (burst discharge) during start-up, while running readings of the residual capacity value are derived afterward from the integration value of charge/discharge current of the battery, while an engine is in operation, or in other words, an a.c. generator is generating electricity.
Yet, if this kind of current integration method is employed, it is necessary to put a current sensor and monitor device in operation. Consequently, when the engine is not in operation, or the generator is not generating electricity, a dark current consumed by the current sensor and the monitor device results in a decreased battery capacity, and that substantially makes it impossible to monitor the battery state while the engine is not running.